Active antenna arrays are expected to provide performance improvements and reduce operating costs of communications systems. An active antenna array includes an array of antenna elements. In this context, the antenna element may be viewed as being a transducer which converts between free-space electromagnetic radiation and guided waves. In an active antenna array, each antenna element, or a subgroup of antenna elements, is associated with an active module. The active module may be a low-noise receiver for low-noise amplification of the signal received by its associated antenna element(s), or it may be a power amplifier for amplifying the signal to be transmitted by the associated antenna element(s). Many active antenna arrays use transmit-receive (T/R) modules which perform both functions in relation to their associated antenna elements. The active modules, in addition to providing amplification, ordinarily also provide amplitude and phase control of the signals traversing the module, in order to point the beam(s) of the antenna in the desired direction. In some arrangements, the active module also includes filters, circulators, and or other functions.
A major cost driver in active antenna arrays is the active transmit or receive, or T/R module. It is desirable to use monolithic microwave integrated circuits (MMIC) to reduce cost and to enhance repeatability from element to element of the array. Some prior-art arrangements use ceramic-substrate high-density-interconnect (HDI) substrate for the MMICs, with the substrate mounted to a ceramic, metal, or metal-matrix composite base for carrying away heat. These technologies are effective, but the substrates may be too expensive for some applications.
FIG. 1 illustrates a cross-section of an epoxy-encapsulated HDI module 10 in which a monolithic microwave integrated circuit (MMIC) 14 is mounted by way of a eutectic solder junction 16 onto the top of a heat-transferring metal deep-reach shim 18. The illustrated MMIC 14, solder 16, and shim 18 are encapsulated, together with other like MMIC, solder and shim assemblies (not illustrated) within a plastic encapsulant or body 12, the material of which may be, for example, epoxy resin. The resulting encapsulated part, which may be termed "HDI-connected chips" inherently has, or the BP an lower surfaces are ground and polished to generate, a flat lower surface 12.sub.ls BP an .sub.us . The flat lower surface 12.sub.ls, and the exposed lower surface 18.sub.ls of the shim, are coated with a layer 20 of electrically and thermally conductive material, such as copper or gold. As so far described, the module 10 of FIG. 1 has a plurality of individual MMIC mounted or encapsulated within the plastic body 12, but no connections are provided between the individual MMICs or between any one MMIC and the outside world. Heat which might be generated by the MMIC, were it operational, would flow preferentially through the solder junction 16 and the shim 18 to the conductive layer 20.
In FIG. 1, the upper surface of MMIC 14 has two representative electrically conductive connections or electrodes 14.sub.1 and 14.sub.2. Connections are made between electrodes 14.sub.1 and 14.sub.2 and the corresponding electrodes (not illustrated) of others of the MMICs (not illustrated) encapsulated within body 12 by means of HDI technology, including flexible layers of KAPTON on which traces or patterns of conductive paths, some of which are illustrated as 32.sub.1 and 32.sub.2, have been placed, and in which the various layers are interconnected by means of conductive vias. In FIG. 1, KAPTON layers 24, 26, and 30 are provided with paths defined by traces or patterns of conductors. The layers illustrated as 24 and 26 are bonded together to form a multilayer, double-sided structure, with conductive paths on its upper and lower surfaces, and additional conductive paths lying between layers 24 and 26. Double-sided layer 24/26 is mounted on upper surface 12.sub.us of body 12 by a layer 22 of adhesive. A further layer 30 of KAPTON, with its own pattern of electrically conductive traces 32.sub.2, is held to the upper surface of double-sided layer 24/26 by means of an adhesive layer 28. The uppermost layer of electrically conductive traces may include printed antenna elements in one embodiment of the invention. As mentioned above, electrical connections are made between the conductive traces of the various layers, and between the traces and appropriate ones of the MMIC contacts 14.sub.1 and 14.sub.2, by through vias, some of which are illustrated as 36. The items designated MT0, MT1, MT2, and MT3 at the left of FIG. 1 are designations of various ones of the flexible sheets carrying the various conductive traces. Those skilled in the art will recognize this structure as being an HDI interconnection arrangement, which is described in U.S. Pat. No. 5,552,633, issued Sep. 3, 1996 in the name of Sharma.
As illustrated in FIG. 1, at least one radio-frequency (RF) ground conductor layer or "plane" 34 is associated with lower layer 24 of the double-sided layer 24/26. Those skilled in the art will realize that the presence of ground plane 34 allows ordinary "microstrip" transmission-line techniques to carry RF signals in lateral directions, parallel with upper surface 12.sub.us of plastic body 12, so that RF signals can also be transmitted from one MMIC to another in the assembly 10 of FIG. 1.
Allowed U.S. patent application Ser. No. 08/815,349, in the name of McNulty et al., describes an arrangement by which signals can be coupled to and from an HDI circuit such as that of FIG. 1. As described in the McNulty et al. application, the HDI KAPTON layers with their patterns of conductive traces are lapped over an internal terminal portion of a hermetically sealed housing. Connections are made within the body of the housing between the internal terminal portion and an externally accessible terminal portion.
One of the advantages of an antenna array is that it is a relatively flat structure, by comparison with the three-dimensional curvature of reflector-type antennas. When assemblies such as that of FIG. 1 are to be used for the transmit-receive modules of an active array antenna, it is often desirable to keep the structure as flat as possible, so as, for example, to make it relatively easy to conform the antenna array to the outer surface of a vehicle. FIG. 2a illustrates an HDI module such as that described in the abovementioned McNulty patent application. In FIG. 2a, representative module 210 includes a mounting base 210, to which heat is transferred from internal chips. A plurality of mounting holes are provided, some of which are designated 298. A contoured lid 213 is hermetically sealed to a peripheral portion of base 212, to protect the chips within. A first set of electrical connection terminals, some of which are designated as 222a, 224a, and 226a are illustrated as being located on the near side of the base, and a similar set of connection terminals, including terminals designated as 222b, 224b, and 226b are located on the remote side of the base. FIG. 2b is a perspective or isometric view, partially exploded, of an active array antenna 200. In FIG. 2b, the rear or reverse side (the non-radiating or connection side) of a flat antenna element structure 202 is shown, divided into rows designated a, b, c, and d and columns 1, 2, 3, 4, and 5. Each location of array structure 202 is identified by its row and column number, and each such location is associated with a set of terminals, three in number for each location. Each array location of antenna element array 202 is associated with an antenna element, which is on the obverse or front side of structure 202. Each antenna element on the obverse side of the antenna element structure 202 is connected to the associated set of three terminals on the corresponding row and column of the reverse side of the antenna element array 202. Each antenna element of active antenna array 200 of FIG. 2b is associated with a corresponding active antenna module 210, only one of which is illustrated. In FIG. 2b, active antenna module 210b3 is associated with antenna element or array element 202b3. Active module 210b3 is identical to module 210 of FIG. 2a and to all of the other modules (not illustrated) of FIG. 2b. Representative module 210b3 has its terminals 222a, 224a, and 226a connected by means of electrical conductors to the set of three terminals associated with array element 202b3 of antenna structure 202. The other set of terminals of module 210b3, namely the set including terminals 222b, 224b, and 226b, is available to connect to a source or sink of signals which are to be transmitted or received, respectively. It will be clear that the orientation of module 210b3, and of the other modules which it represents, will, when all present, will extend for a significant distance behind or to the rear of the antenna element support structure 202, thereby tending to make the active antenna array 200 fairly thick. Also, the presence of the many modules will make it difficult to support the individual modules in a manner such that heat can readily be extracted from the mounting plates (212 of FIG. 2a). Also, the presence of many such active modules 210 will make it difficult to make the connections between the terminal sets of the active modules and the terminal sets of the antenna elements. The problem of thickness of the structure of FIG. 2b is exacerbated by the need for a signal distribution arrangement, partially illustrated as 290. Distribution arrangement 290 receives signal from a source 292, and distributes some of the signal to the near connections of each of the modules, such as connections 222b, 224b, and 226b of module 210b3.
A further problem with the structure of FIG. 2b is that the connections between the active module 210b3 and the set of terminals for array element 202b3 is by way of an open transmission-line. Those skilled in the art of RF and microwave communications know that such open transmission-lines tend to be lossy, and in a structure such as that illustrated in FIG. 2b, the losses will tend to result in cross-coupling of signal between the terminals of the various array elements.
A further problem with interconnecting the structure of FIG. 2b is that of tolerance build-up between the antenna terminal sets on the reverse side of the antenna element structure 202, the terminals of the modules 210, and the terminals of beamformer 290.
Improved arrangements are desired for producing flat HDI-connected structures which can be arrayed with other flat structures.